In recent years, there have been an increased number of vehicles equipped with a curtain airbag in addition to a front airbag and a side airbag. The front airbag is used in the event of a front collision of a vehicle. The side airbag is used in the event of a side collision of the vehicle. The curtain airbag is used in the event of a rollover of the vehicle. Specifically, in the event of the rollover, the curtain airbag deploys to cover a side window of the vehicle, thereby preventing an occupant from being thrown out of the vehicle.
As disclosed, for example, in JP-A-2006-44454, in a vehicle equipped with a curtain airbag, a roll rate sensor is placed almost in the center of the vehicle to detect a rollover of the vehicle.
In an apparatus disclosed in JP-A-2006-44454, rollover determination is performed by using not only a signal from the roll rate sensor but also a signal from a satellite sensor that is placed on each side of the vehicle to detect a side collision of the vehicle. In such an approach, types of rollovers (flipover, turnover, tripover, and the like) can be identified.
Due to diversification of occupant protection devices, the number of sensors mounted on a vehicle has been increased. Accordingly, the cost of the overall system has been increased. In particular, although a front collision and a side collision can be detected by using an acceleration sensor, a roll rate sensor is necessary to detect a rollover. The addition of the roll rate sensor results in an increase in the cost.